Alkali metal ion complexes with benzophenones in rigid glass solution

INORG.

NUCL.

CHEM.

LETTERS

Vol.

12, pp.

345-349,

1976.

Pergamon

Press.

Printed

in

Great

Britain

ALKALI METAL ION COMPLEXES WITH BENZOPHENONES IN RIGID GLASS SOLUTION

C. J. Marzzacco Department of Physical Science Rhode Island College Providence, R. I. 02908 L. H. Zeitlin Department of Chemistry New York University University Heights Bronx, New York ~ e c e ~ 22 Ju~ 1975; m revised~rm 22 December 1975)

INTRODUCTION One of us recently presented a study of the complexation of pyrazine with various alkali metal ions in a rigid ethanol glass at 77°K 1. These complexes were detected by a careful examination of the absorption and phosphorescence spectra of the oyrazine in ethanol with various concentrations of the metal salts. The pyrazine molecule has lowest singlet and triplet excited states of n ~ character and any complexation of pyrazine involving the nonbonding orbitals results in substantial changes in both the singlet- slnglet n -- ~ absorptlon and the triplet - singlet~* -- n phosphorescence spectra. This study clearly demonstrated that luminescent molecules with lowest n~ * excited states are well suited for complexation studies. Since aromatic ketones such as benzophenones are of this type, we decided to investigate the possibility that these molecules complex with alkali metal ions. In this letter we present evidence that benzophenone and 4, 4' dimethoxy-benzophenone (DMB) form complexes with the alkali metal ions in a rigid ethanol glass at 77°K.

EXPERIMENTAL Zone-refined benzophenone, 9 9 . 9 ~ (Columbia Organic Chemicals) was used without further purification and 4, 4' dimethoxy-benzophenone (Aldrich Chemical Co.) was purified by recrystallization. All salts were of reagent grade quality (Mallinckrodt Chemical Co.) and were used without further purification. Absoh~te ethanol (U.S. Industrial Chemical Corporation) was also used without further purification. All solutions were made up fresh, placed in quartz tubes, and rapidly immersed in liquid nitrogen in optical quartz dewars. The emission spectra were taken on a Baird-Atomic Model SF-1 fluorescence spectrometer. The absorption spectra were taken on a Cary 15 spectrometer with the samples placed in contac~ with liquid nitrogen in a quartz dewar. RESULTS AND DISCUSSION Absorption Spectra The 7T°K n-~IT W absorption spectra of DMB in ethanol with various concentrations of KAc are shown in Figure 1. Spectrum 1 is of the ketone in ethanol with 345

Alkali Metal ion Complexes

346

200

18o

120

C 80

4

l

,o

!

0

L_

I

3100

33oo

i

WAVELENGTH Fig. 1

i

35oo

3700

i

39o0

4,

The 77°K absorption spectra of DMB in ethanol with (1) O.OM, (2) 0.20 M,

(3) o.4o M and (4) 0.SO M KAc. no salt and is characteristic of uncomplexed DMB. 2,3 This spectrum has an origin at 3720 A and shows a vibrational progression of bands which correspond to the ll50 cm-1 excited state carbonyl stretching mode. The presence of increasing amounts o~ KAc(spectra 2-4) results in a substantial decrease in the iatenslty of the 3720oA origin band, a less marked decrease in the intensity of the second band at 3565 A and an increased intensity in the shorter wavelength portion of the spectrum. It appears that the first band is disappearing and that the other bands are shifting slightly to longe~ wavelength. In addition, three isosbestic points appear at 3650, 3620 and 3520 A. We interpret these spectral changes as being due to complexation of DMB occurring when the salt is present. It appears that the origin of the eomplexed species lies slightly to the red of the second band of the uncomplexed species. When either of two other soluble potassium salts, KBr or KI, is used in place of KAc identical spectral shifts and isosbestic points are observed Thls indicates that K+ rather than the anion is responsible for the complexation. From the magnitude of the spectral changes it appears that the complexation must occur at the nonbonding orbital on the oxygen. Emission Spectra The 7V°K phosphorescence spectra of the DMB - KAc system provide additional support for complexation. Figure 2 shows the phosphorescence spectra of solutions which are all 1.O x lO-3M inoDMB and are O.OM,.40 M and .80 M in KAc. These spectra were all excited at 3720A, the singlet origin of the uncomplexed ketone. Hence, only the uncomplexed species gets excited. We see that increasing

Alkali Metal Ion Complexes

347

z LIJ I--Z

I

I

4100

4300

I

I

I

I

4500 4700 4900 WAVELENGTH A

5100

|

5300

Fig. 2 The 77°K phosphorescence of DMB in ethanol with (a) O.OM, (b) 0.40M, and (c) 0.80 M KAc excited at 3720A. concentrations of KAc cause the phosphorescence intensity to diminish but results in no changes in the band positions or the Franck-Condon patterns of the spectrum. This spectrum results from the triplet n--,* emission of uncom?lexed DMB. It has an origin at 4186~ and shows a progression of the 1600 cm-± ground state carbonyl stretching mode. The fact that the intensity is diminished by the presence of KAc is interpreted as due to the complexation of the ketone with K+. This complexation reduces the concentration and therefore the emissSon intensi~y of the uncomplexed species which is the only species that gets excited at 3725A. Quite a different result, however, is observed when the phosphorescence is excited at a wavelength short enough to excite both the complexed and the uncomplexed species. Figure 3 shows the phosphorescence spectrum of a ser~es of solutions similar to those used in Figure 2 but with excitation at 3460A rather than 3720A. Spectrum 1 is of DMB in ethanol with no KAc. This is the same spectrum that was seen in Figure 2 and is characteristic of the uncomplexed species. The presence of KAc (spectra 2-4), however, results in a substantial change in the phosphorescence spectrum.

We n o t i c e

that

there

is q u i t e

a reduction in the intensity of the uncomplexed DMB bands at 4190, 4500, 4830 and ~230~ and a growth of new sub-spectrumwith bands at 4110, 4390, 4710 and 5080A. This new subspectrum is interpreted as due to the complexed ketone. It is clearly similar to the uncomplexed ketone spectrum, showing a progression of the carbonyl stretching mode with a similar Franck-Condon pattern. The main difference between the two spectra is that the complexed ketone spectrum is shifted by 460 cm-1 to higher energy relative to the uncomplexed spectrum. This shift is similar to what has previously been observed for ketones in a mixed solvent of ethanol and water. 2 In that system two hydrogen bonded species were observed. The spectrum of the species with a strong hydrogen bond to water is also shifted to higher energy relative to the species with a weak hydrogen bond to ethanol. This occurs because the ground state has a greater nonbonding electron density than the n--,* state. As a result of this the replacement of a weak hydrogen bond with ethanol by a strong hydrogen bond w i t h w a t e r produces a greater stabilization of the ground state than the n-~,* state and therefore a

Alkali Metal Ion Complexes

348

1

4,

4

I

I

IZ 1.1.1 Iz

i

4100

4:500

I

I

4500

4700

I

4900

I

I

5100

5300

WAVELENGTH

Fig. 3 The 77°K phosphorescence of DMB inAethanol with (i) O.OM, (2) 0.20M, (3) 0.hOM and (4) 0.80M KAc excited at 3460~.

greater n-- ~ * transition energy. It is apparent that metal ions act in a similar way. The replacement of a hydrogen bond with ethanol by a stronger bond with the metal also results in a blue shift. The f2ct that the metal complexed subspectrum does not appear when excitation is at 3725)[ (Figure 2) occurs because at that wavelength only the uncomplexed DMB gets excited and during its triplet lifetime it must not have a chance to get complexed. This indicates that the rate of equilibration of excited DMB and the metal ion is slow compared to the 14.5 ms triplet lifetime of DMB 2. Strong evidence for complexation of DMB with Li* and Na* and of benzophenone with Li*, Na+ and K + was also found. The spectral changes observed in these cases were similar but by no means identical to those present for the DMB-K+ systems. The apparent stability constants for these various systems have been determined by measuring the relative phosphorescence intensity of the uncomplexed ketone in a series of solutions all with the same ketone concentration but with various metal ion concentrations. In these measurements an excitation wavelength was chosen such that only this species would get excited. For a given solution with metal ion present, the phosphorescence intensity will be proportional to the concentration of the uncomplexed species and the amount by which the phosphorescence intensity is reduced relative to the solution with no metal ion present will be proportional to the concentration of the complexed species. Therefore, for each solution we can get the concentration of each species and the concentration of the uncomplexed metal ion from which we can calculate the stability constant.

Alkali Metal Ion Complexes

349

In general it was found that within experimental error the apparent stability constants did not change with the concentration of the metal present. However, in the case of KAc the apparent stability constant decreased significantly with increasing salt concentration. This result is interpreted as being due to significant ion pairing taking place in this system. In fact these ketones can serve as excellent probes for the study of ion pairing. The apparent stability constants are shown in Table I. We find that the stability constants for DMB are all considerably larger than those of benzophenone reflecting the electron releasing power of the methoxy groups. In addition the stability constants for both ketones increases with increasing atomic number of the alkali metal ion. At first thought one might expect just the opposite behavior since the strength of the metal complex bond should increase with charge density of the metal ion. However, the stability of the solvated metal ion must also be considered since the more stable the solvated ion the smaller will be the stability constant. This stability will also increase with the charge density of the metal ion. TABLE 1 Stability Constants for Benzophenone and 4,4'-Dimethoxybenzophenone with Various Metals

+

+

Ks =

+] B]

Metal Li+(LiCl)

K

for Benzophenone s(i/mole) .12

!

K s for 4,4'-Dimethoxybenzophenone

(1/ ole)

.04

.54

~

.o5

Na+(NaBr)

1.3

i

.2

3.2

+-

.i

K+(~Ac)

2.6

!

.2

4.0

+_

.2

Finally it should be mentioned that it is quite likely that these systems are not in dynamic equilibrium at 7T°K due to the rigid nature of the solution. These systems may achieve equilibrium at some higher temperature and then get locked in as the temperature drops.

Acknowledgement Gratitude is expressed to the Rhode Island College Faculty Research Committee for a grant which was used to support this project.

REFERENCES 1

C. Marzzacco, J. Phys. Chem.

To be published

2

E° Malawer and C. Marzzacco, J. Mol. Spectry 46, 341 (1973).

3

R. G. Lewis and J. J. Freeman, / .

Mol. Spectry 32, 24 (1969).